While no longer the largest volume vehicles in coatings, alkyd coatings are still of major importance since they are the most commonly used resin or binder system in oil-based and solvent-based coatings. Alkyd coatings are relatively inexpensive and perform well, often with fewer film defects than other coatings. They are used in many industrial and architectural applications. The hydrophobic nature of the alkyd polymer makes them good choices when water repellency is important.
Alkyd resins are polyesters generally prepared from a polyol, phthalic anhydride, and unsaturated vegetable fatty acids such as linseed, soy, or twig oil. The inclusion of the fatty acid confers a tendency to form a flexible coating. Alkyds are often categorized as long, medium, or short oil based on the amount of vegetable oil in the alkyd; long oil alkyds have more fatty acid content than short oils.
Preferred fatty acids are those known as drying oils with multiple double bonds since they will air cure to give a hard coating. This curing reaction crosslinks the oligomeric alkyd chains to build molecular weight and improve durability and other properties. Alkyds are sometimes modified with other radical reactive monomers or polymers for a number of reasons. These are included to speed curing, to improve water compatibility or solubility, or to lower viscosity.
Solvents are employed in traditional alkyd manufacture to reduce the viscosity of the oligomeric polyester and often to help remove byproduct water formed in the synthesis. These solvents include xylene or ketones. However, these solvents are classified as VOC, volatile organic compounds. In recent years, the U.S. EPA has passed stringent regulations mandating significant reductions of VOCs in alkyd coatings. Additional restrictions of VOC in coatings will be enacted in the U.S. The European Community mandated that solvent borne alkyd coatings be limited to 50 g/l VOC by 2010, thus effectively eliminating solvent borne alkyd coatings in that area.
There are several types of alkyds. The main classification is into oxidizing and nonoxidizing types. This invention is mainly concerned with the oxidizing types. Oxidizing alkyds cross-link by the same mechanisms as drying oils crosslink, that is, cross-linking through double bonds and preferably through conjugated double bonds.
A number of new technologies have been recently developed to render solvent borne alkyd coatings more environmentally acceptable by replacing some or all of the solvent with water. Alkyd resins are converted into useable waterborne products by one of two methods. One method is to graft an alkyd resin onto a latex polymer. This gives the coating properties of both polymer types. The latex polymer and the surfactant additives that are used to manufacture the latex render the latex/alkyd polymer dispersible in water, while the grafted alkyd renders alkyd-type properties such as toughness and resistance to various chemicals to the coating.
Additional processing steps are needed to make these hybrid products. The alkyd polymer needs to be of a particular type and structure in order to make a viable alkyd/latex coating. In some cases, the durability of these products is superior to those of latex polymeric coatings, but the alkyd resin chemistry must be altered to maximize the benefits of the grafted alkyd resin onto the latex backbone. These additional processing steps add cost to the product. This process also employs additives such as coalescing solvents to improve properties such as the gloss and flexibility of the coating.
Emulsification of the alkyd resin into water is the other method to remove some or all of the VOCs in the emulsified product. One of the main advantages of the emulsification process is that the alkyd resin used in this application does not necessarily need to be altered to prepare the emulsion, as long as the proper surfactant and emulsification process is used to make the product. The proper surfactant is one with the proper molecular weight, structure, and HLB (hydrophile/lipophile balance).
A number of emulsification processes are known. U.S. Pat. No. 6,780,910 Bouvy et al. describes methods to prepare alkyd emulsions. In Zukert et al. U.S. Pat. No. 3,979,346, it is proposed to prepare aqueous dispersions of alkyd resins by the use of a hydrophilic polyoxyethylene non-ionic emulsifier containing two or more unsaturated fatty alcohol or fatty acid groups, together with an anionic surfactant containing carboxylic acid groups prepared from a drying oil and maleic anhydride, which is hydrolyzed in the process. The properties of such dispersions are far from optimal, and coatings prepared therefrom may absorb water and hydrolyze. McNamee et al. in U.S. published application US 2007/0299228 disclose the use of branched polyoxyalkylene surfactants modified by reaction with an unsaturated fatty acid to contain more than one unsaturated fatty acid group. Preferred are fatty acid reaction products of polyoxyethylated sugars such as sorbitol. Due to the hydrophilic nature of the surfactant, water resistance of coatings prepared therefrom may be compromised.
There are a number of technical difficulties and limitations with the emulsification process as described in the patent literature. Usually an invert emulsion process is preferred since it is less capital intensive because it does not require high shear mixing equipment and is also easier to process since the inversion process produces less foam than shearing into water.
In the invert emulsion process, the neat alkyd with or without solvent is heated to a high enough temperature to reduce its viscosity to a manageable level. The surfactant package of choice is then added to the molten alkyd, followed by the gradual addition of hot water. As the water is added the mixture forms a water-in-oil emulsion, but as the water content increases and the emulsion nears the inversion point, “flipping” from a water-in-oil emulsion to an oil-in-water emulsion, the viscosity often becomes unmanageably high. The temperature is maintained as high as possible to reduce the viscosity of the emulsion, but this can cause problems since nonionic surfactants have lower water solubility as the temperature increases. Once the inversion point is crossed, the viscosity drops.
The solvent used to make the alkyd is often removed prior to emulsion preparation since if it is left in the emulsion it contributes to VOCs in the coating based on the waterborne alkyd. Also, if it is left in with the alkyd in emulsion preparation its later removal is often difficult or impossible since the emulsion stability is negatively affected. However, the solvent often must be left in with the alkyd emulsion to keep the viscosity at a manageable level throughout the inversion process. With solvent present through the emulsion process the alkyd does not have to be heated to as high a temperature to keep it fluid or from solidifying. Thus an emulsification method that allowed the alkyd solvent to remain through the inversion process so that lower viscosity could be achieved at lower temperatures and thus shorten production times and reduce cost, but be able to be later removed to produce a low or no VOC emulsion would be desirable.
These alkyd emulsions contain a few percent of surfactants that remain in the formulation and thus are present in the coating after application. There they can cause a number of problems. If the surfactant molecules remain unbound and free to migrate, when the dried coating later is exposed to water some of the surfactant molecules can dissolve into the water. This reduces the surface tension of the water, improves its wetting of the coating surface, and promotes its penetration through the coating. When surfactant-coated micelle spheres stack on a surface during the drying process and start to coalesce, the concentration of surfactant molecules at the intersection of micelles can become relatively high. If these surfactant molecules are not bound to the alkyd resin, upon exposure to water extraction of the surfactant takes place leading to pitting and degradation of the integrity of the coating. Damage to the substrate to be protected becomes much more likely. On metal substrates, this can lead to corrosion and loss of adhesion. In addition, free surfactants in the coating can also plasticize the alkyd and prevent its reaching maximum hardness upon curing.
Many of the problems that are caused by extractable surfactants can be mitigated if the surfactant can co-cure into the alkyd coating. This locks it into place so that it cannot be extracted by water. Therefore reactive surfactants that can co-cure with the alkyd in the air autoxidation process to become locked into the coating and avoid the above mentioned problems caused by free surfactants would be desirable.
A number of patent applications claim reactive surfactants that can do this. They usually are based on fatty acid alkoxylates in which the fatty acid has a number of double bonds similar to the drying oil fatty acids. However, most of the known ones have some significant drawbacks. They are nonionic surfactants and so have lower solubility in hot water, which limits their ability to make inverse emulsions with a manageable viscosity. Nonionics also cannot take advantage of charge stabilization mechanisms to reduce particle size and improve emulsion stability. Most known nonionics for alkyd emulsions have long ethoxylate chains to give them a high HLB which is required to make a stable emulsion. These nonionics have a strong tendency to slow drying, likely due to complexation of metal ion drying agents. Long chain nonionics also have a tendency to plasticize alkyd coatings, reducing their hardness.
The previous alkyd reactive surfactants contain hydrophobes designed to have good compatibility with the fatty acid chains of the alkyd resin; the surfactants of the instant invention were designed to have good compatibility with the aromatic groups of the polyester portion of most alkyd resins.